Cleanup registration of closures

pkg/compiler/lib/src/universe/codegen_world_builder.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of world_builder;
 
 /// World builder specific to codegen.
 ///
 /// This adds additional access to liveness of selectors and elements.
 abstract class CodegenWorldBuilder implements WorldBuilder {
@@ skipping 387 matching lines @@
         break;
       case StaticUseKind.DIRECT_INVOKE:
         _MemberUsage instanceUsage =
             _getMemberUsage(staticUse.element, memberUsed);
         memberUsed(instanceUsage.entity, instanceUsage.invoke());
         _instanceMembersByName[instanceUsage.entity.name]
             ?.remove(instanceUsage);
         useSet.addAll(usage.normalUse());
         break;
     }
-    memberUsed(usage.entity, useSet);
+    if (useSet.isNotEmpty) {
+      memberUsed(usage.entity, useSet);
+    }
   }
 
   void processClassMembers(ClassElement cls, MemberUsedCallback memberUsed) {
     cls.implementation.forEachMember((_, MemberElement member) {
       assert(invariant(member, member.isDeclaration));
       if (!member.isInstanceMember) return;
       _getMemberUsage(member, memberUsed);
     });
   }
 
@@ skipping 68 matching lines @@
         return true;
       }
       return false;
     }
 
     while (cls != null && processClass(cls)) {
       cls = cls.superclass;
     }
   }
 }